1. Field of the Invention
The present invention relates to image an forming method and apparatus for generating recording liquid having a predetermined density and/or a predetermined color by changing a proportion of mixture of image forming liquid and clear liquid based on an image signal and leading the thus-obtained recording liquid to an image receiving medium to form an image.
2. Description of the Related Art U.S. Pat. No. 3,416,153 (which will be referred to as a prior art reference 1, hereinafter) discloses an image forming method, in which a series of charged ink droplets having predetermined intervals is caused to pass through an electric field modulated by an image signal. Unnecessary ink droplets are deflected to be removed and desired ink droplets are selectively guided to a recording sheet so as to form an image on the recording sheet. Since the ink droplets are continuously ejected or jetted in this system, this is referred to as a continuous ink jet system.
U.S. Pat. No. 3,946,398 (which will be referred to as a prior art reference 2, hereinafter) discloses a recording method, in which a piezoelectric transducer plate is deformed by a modulation of an image signal to push out the ink. The pushed-out ink droplets are jetted or expelled from an orifice to be impacted on a recording medium. This system is referred to as a piezo ink jet system.
U.S. Pat. No. 4,490,728 (which will be referred to as a prior art reference 3, hereinafter) discloses another recording method, in which the ink is rapidly expanded or vaporized by heat of a heater modulated by an image signal. The rapidly-expanded ink gas or vapor is used to jet the ink liquid from an orifice to be impacted on a recording medium. Since ink droplets are jetted by using heat, this is referred to as a thermal ink jet system.
U.S. Pat. No. 4,109,282 (which will be referred to as a prior art reference 4) discloses a printing device, in which a valve called a flap valve is provided in a flow path for leading two types of liquid, i.e., clear ink and black ink into a substrate for forming an image. The flow path for each ink is opened/closed by displacement of this valve so that the two types of liquid are mixed in a desired density to be transferred onto the substrate. This enables printout of an image having the gray scale information which is the same with that of the image information displayed on a TV screen.
This reference 4 discloses that a voltage is applied between the flap valve and an electrode provided on a surface opposed to the flap valve and the valve itself is mechanically deformed by the electrostatic attracting force to cause displacement of the valve. Further, the ink is absorbed in paper by a capillary action which acts on the ink between a tip of the flap valve and fibers of the print paper.
Unexamined Japanese Patent Publication (KOKAI) No. 291663/1988 (which will be referred to as a prior art reference 5, hereinafter) discloses a coating method, in which two types of thick (dark) and thin (light) liquid are mixed in a coating head to be continuously extruded from a slot-opening opposed to a running web. Thus, the mixed liquid is consecutively coated on the web. In this coating method, the mixed liquid is coated over the entire coating width with a uniform coating membrane pressure without forming a residue deposit, and the coating liquid having a density graduation in time course is continuously applied with respect to a traveling direction of the web. In addition, this method enables coating with a uniform thickness with respect to the width direction.
According to the method disclosed in the prior art reference 1 (the continuous ink jet system), unnecessary ink droplets are removed by modulating the electric field to enable drawing of a desired image. However, it is required to provide each mechanism for independently modulating the electric field for each nozzle provided for each pixel, thereby making it difficult to reduce the dimension of each nozzle. It is also hard to form multiple nozzles with a high density in accordance with pixels. Only a part of continuously jetted ink droplets must be used for forming an image, and hence this mode is not suitable for high speed recording because many ink droplets are not used but removed. Moreover, since the ink is continuously jetted, a large amount of ink is wasted, and the obtained print is thus expensive.
According to the method disclosed in the prior art reference 2 (the piezo ink jet mode), a desired image can be drawn by jetting only ink droplets which are used for forming an image. Jetting only a necessary amount of ink eliminates the waste of ink, and a relatively-inexpensive print can be obtained. However, the nozzles must be arranged in the high density for realizing the high quality of an image, leading to a problem where the image is distorted by the interaction of the ink droplets jetted from adjacent nozzles.
According to the method disclosed in the prior art reference 3 (the thermal ink jet mode), an arbitrary image can be drawn and jetting only a necessary amount of ink can obtain a relatively-inexpensive print as similar to the above-mentioned piezo ink jet mode. However, when the nozzles are provided in the high density for realizing the high quality of an image, the image is distorted by the interaction of the jetted ink droplets. Additionally, in the above prior art references 1-3, since the droplets are jetted onto image receiving paper at high speed, a part of the ink droplets smashes by impact to form an ink mist. Such ink mist cannot be captured on the image receiving paper. The uncaptured ink mist leaks to the installation environment of the printer to pollute the environment which is pointed out as a problem.
According to the method disclosed in the prior art reference 4, the ink extruded from the nozzle is directly applied on the paper. Therefore, in a case where the paper has a large thickness or irregularity on the surface of the paper, it is difficult to reproduce an image on the paper with fidelity with respect to the electric signal. Accordingly, this method is not done in practical use, as yet. Further, since the ink to be used is restricted to two types, a color image cannot be recorded. Furthermore, since the ink is drawn out by the capillary action between the ink and the fibers of the paper in this mode, the ink tends to be affected by the quality of the paper and a change in quality of the paper involves a change in quality of an image. Moreover, the image cannot be truly reproduced due to the partial irregularity of the fiber structure even if the paper with the same quality is used.
According to the coating method disclosed in the prior art reference 5, although an image having a density graduation along a traveling direction of a web which is a target of coating can be formed, the image cannot have a density graduation along a width direction of the web (a direction orthogonal to the web traveling direction). Consequently, application of the coating liquid whose color or density changes for each pixel in accordance with an image signal is impossible.
In order to solve the above-described problems in the prior art methods, the present inventors have examined the method for transporting the recording liquid as a continuous flow to the image receiving medium. According to this method, the recording liquid is generated in the recording head by maintaining the overall volume flow rate of the image forming liquid and the clear liquid, which is substantially transparent after dried out, substantially constant while changing a proportion of mixture of these types of liquid based on the image signal, and the recording liquid is applied to the image receiving medium as a continuous flow. In this case, it is required for obtaining the excellent image quality to transport the recording liquid extruded or ejected from the extruding port corresponding to each pixel to the image receiving medium in synchronism with the image signal.
On the other hand, when the recording liquid temporarily pools between the recording head and the image receiving medium to form a bead (a liquid bank), a stream line of the recording liquid may become sinuous in the bead. For example, a turbulence of the recording liquid may be generated in the bead. When such a sinuosity of the stream line is produced, the recording liquid cannot be accurately transported to the image receiving medium in synchronism with the image signal, thus resulting in the distortion of an image or deterioration of the image quality.